The Lithium Flash - Thermal instabilities generated by lithium burning in RGB stars

We present a scenario to explain the lithium-rich phase which occurs on the red giant branch at the so-called bump in the luminosity function. The high transport coefficients required to enhance the surface lithium abundance are obtained in the framework of rotation-induced mixing thanks to the impulse of the important nuclear energy released in a lithium burning shell. Under certain conditions a lithium flash is triggered off. The enhanced mass loss rate due to the temporary increase of the stellar luminosity naturally accounts for a dust shell formation

The Abundance of Interstellar Fluorine and Its Implications

We report results from a survey of neutral fluorine (F I) in the interstellar medium. Data from the Far Ultraviolet Spectroscopic Explorer (FUSE) were used to analyze 26 lines of sight lying both in the galactic disk and halo, including lines to Wolf-Rayet stars and through known supernova remnants. The equivalent widths of fluorine resonance lines at 951.871 A and 954.827 A were measured or assigned upper limits and combined with a nitrogen curve of growth to obtain F I column densities. These column densities were then used to calculate fluorine depletions. Comparisons are made to the previous study of F I by Federman et al. (2005) and implications for F I formation and depletion are discussed.Comment: 32 pages, 10 figures, Accepted to Ap

Fluorine abundances in planetary nebulae

We have determined fluorine abundances from the F II 4789 and F IV 4060 nebular emission lines for a sample of planetary nebulae (PNe). Our results show that fluorine is generally overabundant in PNe, thus providing new evidence for the synthesis of fluorine in asymptotic giant branch (AGB) stars. [F/O] is found to be positively correlated with the C/O abundance ratio, in agreement with the predictions of theoretical models of fluorine production in thermally pulsing AGB stars. A large enhancement of fluorine is observed in the Wolf-Rayet PN NGC 40, suggesting that high mass-loss rates probably favor the survival of fluorine.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

Possible Stellar Metallicity Enhancements from the Accretion of Planets

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central star. We study the ramifications of planet accretion for the final main sequence metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr, stars with masses near 1 solar mass are predicted to have virtually no metallicity enhancement. On the other hand, early F and late A type stars with masses of 1.5--2.0 solar masses can experience significant metallicity enhancements due to their considerably smaller convection zones during the first 10 Myr of pre-main-sequence evolution. We show that the metallicities of an aggregate of unevolved F stars are consistent with an average star accreting about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr dispersal time.Comment: 14 pages, AAS LaTeX, 3 figures, accepted to ApJ Letter

Implications of a Sub-Threshold Resonance for Stellar Beryllium Depletion

Abundance measurements of the light elements lithium, beryllium, and boron are playing an increasingly important role in the study of stellar physics. Because these elements are easily destroyed in stars at temperatures 2--4 million K, the abundances in the surface convective zone are diagnostics of the star's internal workings. Standard stellar models cannot explain depletion patterns observed in low mass stars, and so are not accounting for all the relevant physical processes. These processes have important implications for stellar evolution and primordial lithium production in big bang nucleosynthesis. Because beryllium is destroyed at slightly higher temperatures than lithium, observations of both light elements can differentiate between the various proposed depletion mechanisms. Unfortunately, the reaction rate for the main destruction channel, 9Be(p,alpha)6Li, is uncertain. A level in the compound nucleus 10B is only 25.7 keV below the reaction's energetic threshold. The angular momentum and parity of this level are not well known; current estimates indicate that the resonance entrance channel is either s- or d-wave. We show that an s-wave resonance can easily increase the reaction rate by an order of magnitude at temperatures of approximately 4 million K. Observations of sub-solar mass stars can constrain the strength of the resonance, as can experimental measurements at lab energies lower than 30 keV.Comment: 9 pages, 1 ps figure, uses AASTeX macros and epsfig.sty. Reference added, typos corrected. To appear in ApJ, 10 March 199

The fluorine abundance in a Galactic Bulge AGB star measured from CRIRES spectra

We present measurements of the fluorine abundance in a Galactic Bulge Asymptotic Giant Branch (AGB) star. The measurements were performed using high resolution K-band spectra obtained with the CRIRES spectrograph, which has been recently installed at ESO's VLT, together with state-of-the-art model atmospheres and synthetic spectra. This represents the first fluorine abundance measurement in a Galactic Bulge star, and one of few measurements of this kind in a third dredge-up oxygen-rich AGB star. The F abundance is found to be close to the solar value scaled down to the metallicity of the star, and in agreement with Disk giants that are comparable to the Bulge giant studied here. The measurement is of astrophysical interest also because the star's mass can be estimated rather accurately (1.4 \lesssim M/\mathrm{M}_{\sun} \lesssim 2.0). AGB nucleosynthesis models predict only a very mild enrichment of F in such low mass AGB stars. Thus, we suggest that the fluorine abundance found in the studied star is representative for the star's natal cloud, and that fluorine must have been produced at a similar level in the Bulge and in the Disk.Comment: 11 pages, 1 figure, accepted for publication by Ap

The thermonuclear production of F19 by Wolf-Rayet stars revisited

New models of rotating and non-rotating stars are computed for initial masses between 25 and 120 Msun and for metallicities Z = 0.004, 0.008, 0.020 and 0.040 with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to the F19 enrichment of the interstellar medium. Models with an initial rotation velocity vini = 300 km/s are found to globally eject less F19 than the non-rotating models. We compare our new predictions with those of Meynet & Arnould (2000), and demonstrate that the F19 yields are very sensitive to the still uncertain F19(alpha,p)Ne22 rate and to the adopted mass loss rates. Using the recommended mass loss rate values that take into account the clumping of the WR wind and the NACRE reaction rates when available, we obtain WR F19 yields that are significantly lower than predicted by Meynet & Arnould (2000), and that would make WR stars non-important contributors to the galactic F19 budget. In view, however, of the large nuclear and mass loss rate uncertainties, we consider that the question of the WR contribution to the galactic F19 remains quite largely open.Comment: 9 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

Fluorine Abundances in the Orion Nebula Cluster

This study uses cool dwarfs as sources with which to probe fluorine abundances via HF. This molecule is detected for the first time in young K-M dwarf members of an OB association. Fluorine, oxygen, and carbon abundances were derived from the HF(1--0) R9 line along with samples of OH and CO vibration-rotation lines present in high-resolution infrared spectra observed with the Phoenix spectrograph on the Gemini South Telescope. The fluorine and oxygen results obtained for these targets, still in the pre-main-sequence stage of evolution, agree well with the general trend defined for the Milky Way disk; the latter being deduced from observations of more evolved giant stars. In addition, the carbon and oxygen abundances obtained for the studied stars overlap results from previous studies of the more massive OB stars and FG dwarf members of the Orion Nebula Cluster. We conclude from this agreement that the fluorine abundances derived for the Orion K-M dwarfs (when there is no conspicuous evidence of disks) can be considered a good representation of the current fluorine abundance value for the Milky Way disk.Comment: 18 pages, including 2 tables and 3 figures. Accepted for publication in The Astrophysical Journa